Theory

Stress corrosion cracking (SCC) is a phenomenon which occurs under conditions of constant applied stress for particular combinations of alloy/ material and environment. It is usually sensitive to temperature as well as environmental species and concentration. Mechanisms of SCC are complex and, fractographically, may show either intergranular cracking or cleavage.  More detailed information on SCC can be obtained from the references below, or directly accessed from the internet. Several typical sites which deal with SCC are the UMIST Corrosion Information Server, Intercorr.com which is a commercial corrosion and materials information resource that currently allows some free access, and Corrosion Engineering at NASA's Kennedy Space Centre. The static stress may be either applied by external loads or, as is often the case, arise from residual stresses associated with welding or cold working. Classic examples of SCC are brass in an ammonia containing environment (so-called 'season cracking' which was noted in brass cartridge cases stored next to stables during the monsoon season in India), chloride-induced SCC in austenitic stainless steels and aluminium alloys, some ceramics, glasses and polymers in moist air, and steels in caustic, hydrogen containing or hydrogen sulphide environments.

The area of interest in this theory card is the characterisation of crack growth under SCC conditions by the fracture mechanics parameter K. Interest in the application of fracture mechanics to SCC testing arose because it was realised that a number of alloy-environment combinations which appeared immune to SCC when tested as smooth specimens, were very susceptible to this phenomenon in the presence of a crack or crack-like defect. Hence fracture mechanics tests are used to characterise crack velocity in SCC and find the threshold for stress corrosion crack growth, which is termed K1SCC. As the applied load is constant in SCC, it is more useful to talk about crack velocity and plot these against applied stress intensity level to give what are termed v-K curves. A typical v-K curve is shown below for the case of inorganic glass in a moist air environment (50% relative humidity and a temperature of 25C) and, as is the case for fatigue crack growth, 3 distinct regions can often be observed which reflect the operation of different influences in the mechanisms of cracking.
In the first region the crack velocity increases sharply with increase in applied K as the value of K controls the environmental reaction rate at the crack tip. In the second, more horizontal region, the rate controlling step is environmental transport to the crack tip, which is independent of applied K. In region 3, cracking is again mechanically controlled and K is tending towards the value of the fracture toughness. The steep slope of the curve in region 1 allows a threshold for crack growth K1SCC to be defined, below which growth is essentially non-existent. In the curve above, K1SCC is approximately 2 MPa m as the crack velocity is less than 10-12 m/s. The extent of the regions is variable, and region 1 often dominates the life. As the curve in this region is linear on a log-log plot, it has a simple equation which is easy to integrate to obtain a life estimate. The equation of a straight line is:

It is often useful to use the K1SCC data and recast the equation as:

Separating the variables and integrating this between K limits is straightforward:

The lower limit on the integration would be the K value corresponding to the combination of initial defect size and applied stress, while the upper limit could be either the fracture toughness KC or an upper limit on the extent of region 1.  Typically therefore, separating the variables gives:

Note that if the integrand has n = 2, the expression for tf will contain natural log (ln) terms.

References
  1. American Society for Materials (1996), Stress Corrosion Cracking and Hydrogen Embrittlement ASM Handbook, Vol. 19 Fatigue and Fracture, p.483-506.
  2. American Society for Materials (1992), Stress Corrosion Cracking - Materials Performance and Evaluation, ed. R H Russell.

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